1. Field of the Invention
This invention relates to a method of monitoring a semiconductor process, and more particularly to a method of detecting an end point of a plasma etching process.
2. Description of the Related Art
As the integration degree of semiconductor device is required higher, the control over the plasma etching utilized in the semiconductor process becomes more important. The stop of a plasma etching process may be determined in a time mode, wherein the period of the etching process is fixed to a constant value predetermined based on the experiences. However, since the etching/wafer conditions are rarely constant, a fixed etching period easily causes incomplete etching or too much over-etching.
Another way to determine when to stop a plasma etching process is based on the optical emission spectroscopy (OES) of the plasma. Such a method typically includes monitoring the intensity of one spectral line of one etching product of the layer under the etching target layer in the emission spectrum of the plasma. The time-variation of the intensity reveals when the target layer is etched through to expose the underneath layer, so that a proper end point can be detected for the plasma etching process.
In a modified version of the above method, the intensity of one spectral line of one etching product of the underlying layer and that of one spectral line of one etching product of the target layer are measured and their ratio is calculated in real time. The plot of the ratio with the time shows an enhanced etching-through signal.
However, when there are two or more areas having different etching end-points on a wafer, not all of the end points can be detected with the above methods. FIG. 1 shows the results of respectively applying the two conventional methods of detecting an etching end-point to an exemplary plasma etching process for defining wider holes and narrower holes. The etching process was for simultaneously defining contact holes of 0.16 wide and contact holes of 0.23 μm wide in an inter-layer dielectric (ILD) layer of silicon oxide of 9500 Å thick between an overlying bottom anti-reflection coating (BARC) of 900 Å thick and an etching stop layer of silicon nitride of 200 Å thick. The etching product of silicon nitride being selected was CN, and the spectral line of CN at 387 nm was monitored. The etching product of the silicon oxide being selected was CO, and the spectral line of CO at 483 nm was monitored.
It is known the contact holes of 0.16 μm are opened later than those of 0.23 μm. However, the curve of I387(t) or I387(t)/L483(t) shows only one etching-through signal that allows the etching end-point of the 0.23 μm contact holes to be detected, while the end-point of the 0.16 μm contact holes that is truly important to the plasma etching process cannot be detected because the area percentage of the 0.16 μm contact holes is low.
Therefore, with the conventional end-point detection methods, the degree of the over-etching can be precisely controlled to sustain reasonable consumption of a thin photoresist layer formed in an advance process. Meanwhile, the etching recipe switch timing cannot be well controlled to get a higher selectivity and a better etching profile.
Moreover, as the above end-point detection methods are applied to an etching process for forming contact holes over shallow-junction S/D regions, the etching-through signal is not clear. This makes the end point difficult to detect, so that the Si-loss of the S/D regions cannot be well controlled and the junctions may be damaged.